Directional drilling refers to a type of drilling where a drill tool is directed along a predetermined path by an operator located at a boring machine. By guiding the drill tool from the drilling site, cabling, pipes, or other underground conduits may be installed with minimal disruption to the surface above the location where the borehole is being drilled. Directional drilling techniques have therefore become especially prevalent where there are obstacles on the surface that would make trenching or other conventional installation techniques impossible. For example, directional drilling techniques are especially advantageous when constructing a horizontal borehole beneath a body of water, a roadway, or buildings. Because directional drilling can proceed without regard to surface structures, it has become the chosen technique for many applications.
In many directional drilling techniques, directing a drill tool on a predetermined course is an iterative operation consisting of a locating phase and a drilling phase. During the locating phase, the drill is halted and the position and orientation of the drill tool is determined using one of several methods described in more detail below. Based on the location of the drill tool, an operator can calculate how close the drill tool is to a predetermined borehole path. If the drill tool is to continue along a straight path, the drill stem and drill tool are rotated during the subsequent drilling phase. When the drilling tool is rotated and additional sections of pipe added to the drill stem, the borehole is advanced along a generally straight line. If the drill tool must change direction to bring the borehole back to the predetermined path, the orientation of the drill tool must be determined. In one type of directional drilling system, the connection between the drilling head and the drill collar is slightly bent. The orientation of the bent connection determines the direction that the drill tool will advance when forward pressure is applied and the drill string is not rotated. To direct the course of the borehole back to the predetermined course, the drill head must therefore be turned to the necessary orientation so that forward pressure applied to the drill tool will correct the path of the drill tool. In the subsequent drilling phase, the drill stem is not rotated and the drilling tool advanced. By selectively orienting the bent connection, an operator can therefore steer the drilling tool in a desired direction along a selected path. It will be appreciated that other types of directional drilling systems exist that do not rely on a bent connection to determine the path of the drill tool. Regardless of the mechanism for orienting and steering the drill tool, however, most directional drilling systems incorporate a locating phase and a drilling phase during the directional drilling operation.
Several different techniques are known in the art for determining the location of a drill tool along a predetermined path. For example, U.S. Pat. No. 4,875,014 to Roberts et at. discloses a system that uses a current-carrying grid to assist in locating and guiding a drill head. The current-carrying grid is initially placed above the desired borehole path. The drill head contains a three-axis magnetometer and a three-axis accelerometer. As the drill head proceeds underneath the current carrying grid, components of the magnetic field generated by the grid are detected by the three-axis magnetometer. Using the orientation of the drill head as determined from the three-axis accelerometer signals, the measured magnetic field vector may be transformed into the coordinate system used by the current carrying grid. By comparing the measured magnetic field vector with a number of calculated magnetic field vectors within the grid, the location of the drill head within the grid may be determined. An operator may then steer the drill head along a predetermined path by periodically checking the location of the drill head.
A more common technique for determining the location of a drill tool during horizontal boring uses a triaxial accelerometer to detect a rotation of the drill tool with respect to the gravitation force vector and a triaxial magnetometer to detect components of the Earth's magnetic field. Each of these sensors is placed in the head of the drill, where they are connected to the surface by a cable that carries power and communications. Each accelerometer is sensitive to a component of the rotation of the drill tool and produces a signal proportional to the rotation. From the electrical signals produced by the accelerometers, a system can calculate both the inclination and roll of the drill tool. Each magnetometer produces a DC voltage that is proportional to the magnitude of the earth's magnetic field component that is normal to the pickup coil in the magnetometer. As the drill tool is advanced, the signals from the magnetometer may therefore be used in conjunction with the accelerometer signals to determine the heading or azimuth of the drill tool based on the changes in the magnetic field. By integrating the inclination and azimuth of the drill tool with respect to the distance traveled, the approximate location of the drill tool may be determined. The triaxial accelerometer and triaxial magnetometer combination therefore allow an operator to roughly track the location of a drill tool as the drill tool is advanced. An advantage of using a triaxial accelerometer and magnetometer is that the locating technique is quick, and the hardware is robust and readily available.
While the use of triaxial magnetometers and accelerometers allow an operator to roughly follow a predetermined boring path, the accuracy of the resulting path is not perfect. In order to calculate the location of the boring tool, the signal from the accelerometers must be integrated over the distance traveled by the drill tool. Integrating the accelerometer signals introduces errors into the calculated position of the drill tool. Although the errors may be small for each individual position determination, the cumulative effect of the errors can be great. During the locating phase of directional drilling, errors introduced into the calculated position of the drill tool are added to and magnified by prior errors in measurement. As a result, over long drilling paths, the calculated drilling path may diverge from the desired drilling path by a significant distance when the drill tool reaches the exit point.
Although these errors have generally been acceptable in less demanding drilling applications, in certain applications errors as small as a few feet over the borehole path may be damaging to the success of a project. For example, gravity sewers rely upon a slight grade in the sewer to ensure that all sewage is fed without pumping to a desired destination. The exit point of a borehole used to install gravity sewers must also be calculated and produced with a high degree of accuracy. The installation of gravity sewers using only triaxial magnetometers and accelerometers to determine the path of the drill tool is therefore a risky proposition. The errors introduced into the resulting path due to the integration of the accelerometer signals can cause the path to deviate from the predetermined course to such an extent that the gravity flow of the sewers is impaired or the exit point inaccurate. In demanding applications where the desired drilling path must be accurately followed, it is therefore desirable to improve upon the general method of tracking and directing a drill tool by relying on accelerometer and magnetometer signals.